1. Field of the Invention
The present invention relates to a non-reducible dielectric ceramic composition and, more particularly, a non-reducible dielectric ceramic composition used as a dielectric material for monolithic ceramic capacitors employing a base metal such as nickel as a material for internal electrodes.
2. Description of the Prior Art
As a dielectric material for monolithic ceramic capacitors, there have been used those mainly comprising barium titanate. Such dielectric ceramic compositions of the prior art have the a property that they are reduced considerably when fired in a neutral or reducing atmosphere with a low partial oxygen pressure, resulting in occurrence of semiconductorization of the ceramic composition. For this reason, it is required to use noble metals (e.g., palladium, platinum, etc.) which do not melt even if subjected to a high temperature at which a dielectric ceramic material sinters, and does not oxidize even if fired in an atmosphere with a high partial pressure of oxygen which does not semiconductorize the dielectric ceramic material.
However, use of such noble metals becomes a barrier to cut down the production cost of monolithic ceramic capacitors. For example, the cost of internal electrodes occupies about 30 to 70% of the production cost of monolithic ceramic capacitors.
For the reasons mentioned above, it is preferred to use a base metals such as Ni, Fe, Co, W and Mo. However, if such base metals are used as a material for internal electrodes, and fired in the conventional firing conditions of the dielectric ceramic materials, they oxidize easily and loose functions as the internal electrodes. Thus, in order to use such a base metal as a material for internal electrodes of monolithic ceramic capacitors, it is required to use a dielectric ceramic material which is never semiconductorized even if fired in a neutral or reducing atmosphere with a low partial pressure of oxygen, and which has a sufficient specific resistance and good dielectric properties.
To meet such requirements, there have been proposed some nonreducible dielectric ceramic materials such as, for example, those having a composition of a BaTiO.sub.3 --MnO--MgO--Rare earth element system disclosed in JP-A-63-103861, or those having a composition of a BaTiO.sub.3 --(Mg, Zn, St, Ca)O--B.sub.2 O.sub.3 --SiO.sub.2 system disclosed in JP-B-61-146110.
However, the nonreducible dielectric ceramic composition of JP-A-63-103861 is of no practical use as the insulation resistance and the temperature coefficient of dielectric constant are affected (sensitive) by grain size of the main component BaTiO.sub.3, thus making it difficult to control the composition to obtain stable dielectric properties.
On the other hand, the nonreducible dielectric ceramic composition of JP-B-61-14611 has a dielectric constant of 2000 to 2800 and thus is inferior in dielectric properties to the conventional dielectric ceramic composition with a dielectric constant of about 3000 to 3500 which has been used as a dielectric material for monolithic ceramic capacitors employing noble metals such as Pd. Thus, such a composition cannot replace the conventional dielectric material for the purpose of reducing the cost of monolithic ceramic capacitors since it has disadvantages in miniaturization and capacitance increment of capacitors.
Further, the temperature coefficient of dielectric constant (T.C.C.) of the above composition is .+-.10% over a range of temperatures ranging from -25.degree. to 80.degree. C., which is based on the capacitance at 20.degree. C. as a standard. Such a composition meets the characteristics B specified by JIS. However, the temperature coefficient of dielectric constant becomes more than 10% at temperatures exceeding 85.degree. C., so that it out of X7R characteristics defined in EIA.
Also, all the nonreducible dielectric compositions of the prior art including the above compositions are low in insulation resistance, which becomes an obstacle to reduce the thickness of dielectric ceramic layers.